Iris Hot Spot

[Robert Double]

Hi,

I have a 35mm adapter that is using a rotating ground glass and I am using a Nikon mount lens. However I am having some strange effects that I do not understand. Not being a master of optics I hope someone might be able to shed some light (no pun intended) :)

Wide open @ 2.8 the adapter works fine. But as soon as I stop it down I get two side effects :

1. I am seeing a hot spot in centre which I have worked out is the iris of the lens. As I stop down you can see a circle becoming smaller and smaller ie. the iris. Why would this be happening? Here are some images, zoomed out from the GG with me stopping down :->

http://www.robertrobertproductions.com/hot/f2.8.jpg
http://www.robertrobertproductions.com/hot/f4.jpg
http://www.robertrobertproductions.com/hot/f5.6.jpg
http://www.robertrobertproductions.com/hot/f8.jpg
http://www.robertrobertproductions.com/hot/f11.jpg

2. As I stop down the rotation of the GG becomes visible, and you start to get a flickering effect. Again, I can't understand why this would happen?

Any help or suggestions on either of these issues would be greatly greatly appreciated :)

Thanks

Robert

[Wayne Kinney]

To get rid of the hotstop (which is a commen problem with these adapters), you need a condenser lens (a Plano Convex , pcx lens), with flat side of the lens facing the ground glass.

This will even out that hotspot you see from your adapter. A 100 FL pcx lens will probably be best, like this:

http://www.surplusshed.com/pages/item/l3385.html

You can also get bad hotspot if the ground glass is too transparent, meaning the light is not diffused enough.

2. As I stop down the rotation of the GG becomes visible, and you start to get a flickering effect. Again, I can't understand why this would happen?

Again this is a commen problem with spinning glass adapters. It means that the glass is not uniformly frosted or that its dirty, meaning some areas of the disk are cleaner/more frosted then others, creating a flickering effect as it rotates.

Are you using the spinning plastic CD method?

HTH, Thanks,
Wayne.

[Quyen Le]

When you use the SLR lens to get the image on the GG, the image's brightness is evenly spread across the GG. However, the camera sitting behind the GG will see the center of the image brighter than the edge. The condenser in front of the GG (Wayne's explaination) will help the camera to look at the image differently (just like looking in the viewfinder of the SLR).

There are 2 ways that I know people using to fix this problem.
1. Cam - Achromat/macro - GG - Condenser - SLR lens
2. Cam - Achromat/macro - Condenser - GG - SLR lens
Wayne's and mine use the second approach (the way SLR camera was built) and I believe G35 uses this too.
I believe Oscar and many other use the first approach.
I don't know what the P+S mini35 uses. Base on the documents that I read, I think the first version of mini35 suffer the same thing, can anybody clarify this for us?
There are some Pros and Cons by using this either way. I don't want to start the war here by listing them out, try it for yourself and experiment it and decide what is the best for you, thanks.

Quyen

[Robert Double]

Hi Wayne / Quyen,

Many thanks for the explanation and advice. This forum is just superb for learning.

Believe it or not, I actually already have a DCX lens between the 35mm and the GG. I wonder if there is a better condenser lens to use, or maybe there are limitations to the DCX on what it can do in spreading the light more evenly? As you can see from the images I posted earlier, there is definate hot spot when stopping down.

Really dumb question; Is it an accepted norm that 35mm adapters have hot-spots, and you just don't stop down with the lens? Obviously if I shoot at 2.8 and zoom in within the hotspot I am fine. But if I stop down then then circular hot spot is visible. I am just wondering if this is one of trade-off's you have to live with, or whether you can devise something to over-come this?

Is it possible to create an adapter with a near zero hot-spot?

Robert

[Quyen Le]

On Wayne's and my videos, we don't have hot spot problem even if we stop down to f16. We don't zoom in, thanks.

Quyen

[Glen Hurd]

Not that I'm trying to start a war or anything, but it seems a little counter-productive to put a PCX between SLR and GG.
It reduces the effective focal length of the SLR lens, which in turn increases DOF.
Not by a huge jump (my 50 mm became something like a 42 mm lens). But it seems like an unnecessary step to me.
My first adapter had the PCXs on either side of the gg, and I had serious vignetting issues. With the single PCX on the video side of the gg, all is much better.
My goal is to have just one PCX in the whole rig, and no macro at all. The less glass, the better . . .
I know Wayne has already mentioned that placing a PCX a short distance from the GG gives great results. There may be a sweet spot, but I've been lucky just by eyeballing it. So try the second method, if you have the time and resources. You may be pleasantly surprised.
Quyen, I'd love to hear your comments on the pluses and minuses of the methods you mentioned. You seem to have a pretty clear grasp of what's going on here.
And if my conclusions are wrong, I apologize in advance. It's just what I'm seeing at this point in my own experiments.

Glen

[Quyen Le]

Glen

It's OK for you to have your opinions. I am selling my adapters here so posting my opinions will do more damage than good. I remember we have a post about a condenser between GG and SLR lens. I was against it but others have different views.

Please post your findings on the adapters you are using. If stopping down causes vignetting, so people here can benefit from it.
Micro35 / M2
G35
Letus35
Mini35
And also if it does, where it start to vignette.
Thanks.

Quyen

[Glen Hurd]

I understand your position. :)
However, I don't own any adapters. I'm going with the "DIY at 10x the cost of buying one" approach.
I spent three nights taking pictures with a digital camera using a GG and two PCX lenses to sandwich the GG, and pondering the results. I wish I knew more about optics (among other things).
First thing I ran into was a vignetting issue that was worse than what I was experiencing with my DVX. I am tentatively concluding that it is related to the "exit pupil" of the SLR lens/GG arangement.
The way I imagine it, the image coming off the ground glass is still effected by the cone of light coming off the SLR lens (its iris) and pre-GG PCX setup. If the glass lens on the receiving side (video or digital still) is larger than that "cone", I expect to get vignetting. If the receiving glass is smaller, I get no vignetting, but a loss of optimum brightness. This would explain why people with smaller lenses rarely complain of having any vignetting issues. It would also explain why smaller f-stops on the SLR side can make the vignetting worse on a recieving camera with a large lens, since the cone shrinks.

However, I only have limited experiments to validate this, and I'm a little confused as to how a diffusion material, such as wax, could be effected by that "cone" since the image should be independent at that point.

What I do know is that my first adapter (homemade, so there is no name) used the PCX GG PCX setup and the vignetting was quite intrusive if I tried to maintain a 24 x 36 mm GG frame.
When I tried removing the pre-GG PCX and pulled the 2nd one off the GG, as Wayne had experienced, I found all my vignetting issues were quickly solved.
I am assuming that the post-PCX acts to expand that cone (and image along with it) which is what eliminates the vignetting since the "exit pupil" has been enlarged. Since it also enlarges the image, the receiving lens doesn't have to be as long in order to capture a clean picture off the GG.
I chose my PCX distance based on how far I needed to enlarge the GG image so as to stay in macro mode on the DVX, without the need for another macro lens.
However, I am experiencing a slight pincushioning at the edge of my frame, which I interpret as meaning that my PCX is too far from the GG at this time. I will be trying a slightly shorter FL this week.
By the way, Quyen, I love the simplicity of your design. You da man!

G

[Quyen Le]

Glen,

The purpose of the condenser between the camera and GG is to change the way that the way the camera see the GG. Instead of seeing it from the center, the camera now see the GG's image by all angles especially the edge to eliminate vignetting. That's why our adapters doesn't have vignetting with most lenses when stop down to f16. I wonder if any other unit use the condenser between the GG and camera? if not, do you see any vignetting when stop down? I try not to put the condenser too far from the GG because of sharp focus issue. The focus won't be that sharp if it's so far away. There are some camera with macro can focus really close without the need of an extra achromat. Thanks.

Quyen